Multi-path, multi-oxide-thickness amplifier circuit

ABSTRACT

An embodiment of a multi-path, multi-oxide-thickness amplifier circuit includes a first amplifier having at least one thin-oxide output transistor, and a second amplifier having at least one thick-oxide output transistor. The first and second amplifiers are connected in parallel with each other between an input terminal and an output terminal of the amplifier circuit. The thin-oxide output transistor has a gate-oxide layer thickness that is less than a gate-oxide layer thickness of the thick-oxide output transistor.

BACKGROUND INFORMATION

Major trends in integrated circuit design include reduction in powersupply voltages and transistor dimensions. The power supply voltagereduction is related to the transistor size reduction, as well as toother measures of circuit performance.

One measure of a transistor that has importance is the thickness of itsgate oxide layer. On a basic level, a transistor includes two dopedsource and drain semiconductor regions spaced apart from each other in adifferently doped semiconductor body region. A conducting gate is formedover the portion of the body separating the source and drain.

The gate oxide layer separates the gate from the body portion over whichit is formed. The transistor operates, at least in part, by applying avoltage to the gate, which excites a charge state in the body underneaththe gate to form a conduction channel.

The gate oxide thickness correlates to performance and operationalmeasures of the transistor. A thinner gate oxide produces a fasterdevice, having a higher bandwidth, or unity-gain frequency f_(T). Athicker gate oxide produces a slower device, having a lower unity-gainfrequency f_(T). However, a thinner gate oxide also reduces the maximumvoltage to which the transistor can be exposed without being damaged,and thus the maximum voltage swing that the transistor can supply in acircuit. A thicker gate oxide, by contrast, can withstand a highervoltage without being damaged, and thus can provide a greater voltageswing. In an exemplary 65 nm minimum-feature-size integrated circuitprocess, successively greater gate-oxide thicknesses result intransistors capable of operating under maximum power supply voltages(and thus providing correspondingly related output-voltage swings) of1.0V, 1.8V, 2.5V, and 3.3V, respectively. However, the thickest oxide,capable of providing the greatest output-voltage swing, has a unity-gainfrequency f_(T) that is only one-tenth as great as that of thethinnest-oxide transistor, which has the least output-voltage-swingcapability.

The tradeoff between speed and output-voltage swing, as embodied as aresult of the performance implications of varying gate-oxidethicknesses, surfaces as a corresponding tradeoff in the design ofcircuits using transistors. For example, use of thin-oxide transistorsin an amplifier results in the amplifier having a high bandwidth, butbeing capable of only a limited output-voltage swing. By contrast, useof thick-oxide transistors in an amplifier results in the amplifierbeing capable of a higher output-voltage swing due to its ability tooperate under a greater power supply voltage, but also having has alower bandwidth.

Thus, a need exists to overcome the limitations on amplifier design, andcircuit design in general, imposed by this tradeoff between bandwidthand output-voltage swing resulting from transistor propertiescorrelating to gate-oxide thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

So that features of the present invention can be understood, a number ofdrawings are described below. However, that the appended drawingsillustrate only particular embodiments of the invention and aretherefore not to be considered limiting of its scope, for the inventionmay encompass other equally effective embodiments.

FIG. 1 is a circuit schematic depicting an embodiment of a multi-path,multi-oxide-thickness amplifier circuit.

FIG. 2 is a circuit schematic depicting an embodiment of a firstamplifier, of the amplifier circuit, having at least one thin-oxideoutput transistor.

FIGS. 3A, 3B, and 3C are circuit schematics depicting embodiments of adifferential pair of NMOS transistors, a pseudo differential pair ofNMOS transistors, and a complimentary pseudo differential pair of NMOSand PMOS transistors, which can be used as amplification stages.

FIG. 4 is a circuit schematic depicting an embodiment of a secondamplifier, of the amplifier circuit, having at least one thick-oxideoutput transistor.

FIGS. 5A and 5B are circuit schematics depicting embodiments of an inputfilter and an output filter of a first amplification path of theamplifier circuit.

FIG. 6 is a circuit schematic depicting another embodiment of theamplifier circuit.

FIG. 7 is a circuit schematic depicting another embodiment of theamplifier circuit.

FIGS. 8A and 8B are cross-sectional views of embodiments of thin- andthick-oxide transistors, respectively.

FIG. 9 is a graph depicting embodiments of the frequency response of thefirst amplifier, the second amplifier, and the amplifier circuit (havingthe first and second amplifiers).

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

An embodiment of a multi-path, multi-oxide-thickness amplifier circuitincludes a first amplifier having at least one thin-oxide outputtransistor, and a second amplifier having at least one thick-oxideoutput transistor. The first and second amplifiers are connected inparallel with each other between an input terminal and an outputterminal of the amplifier circuit, and provide first and second distinctamplification paths, respectively, from the input terminal to the outputterminal. Relative to each other, the first amplification path(including the first amplifier) provides a higher bandwidthamplification, and the second amplification path (including the secondamplifier) provides a higher output-voltage-swing amplification. Thefirst and second amplification paths are configured in the amplificationcircuit to not impede each other's ability to contribute high bandwidthor high output-voltage-swing amplification, respectively.

FIG. 1 depicts an embodiment of the multi-path, multi-oxide-thicknessamplifier circuit 20, which includes a first amplification path 24having a first amplifier 28, and a second amplification path 32 having asecond amplifier 36. The first and second amplification paths 24, 32 aredistinct signal-amplification paths between an input terminal 40 and anoutput terminal 44 of the amplifier circuit 20. The first amplificationpath 24, having the first amplifier 28, provides high-bandwidthamplification for an input signal presented to the input terminal 40 toproduce a portion of the output signal at the output terminal 44. Thesecond amplification path 32, having the second amplifier 36, provideslarge output-voltage-swing amplification for the input signal presentedto the input terminal 40 to produce a portion of the output signal atthe output terminal 44.

The first amplifier 28 is configured to include a thin-oxide outputtransistor (e.g., thin-oxide NMOS and PMOS output transistors 48a, 48bshown in FIG. 2). The first amplifier 28 includes at least one, andoptionally a plurality, of thin-oxide output transistors. The thin-oxideoutput transistors of the first amplifier 28 have at least drainsconnected to an output 60 of the first amplifier 28, and are the onlytransistors of the first amplifier 28 having drains connected to theoutput 60 of the first amplifier 28. The first amplifier 28 alsoincludes only thin-oxide transistors throughout. Thus, not only are theoutput transistors thin-oxide transistors, but all the other transistorsof the first amplifier 28 are thin-oxide transistors. The configurationof the first amplifier 28 to have only thin-oxide transistors enablesthe first amplifier 28 to be a higher-bandwidth amplifier in comparisonto a similar amplifier having thick-oxide transistors in abandwidth-affecting signal path. Because the first amplifier 28 containsno thick-oxide transistors in any signal path, the first amplifier 28 isable to take full advantage of the higher-bandwidth of thin-oxidetransistors, and not suffer effects from the lower bandwidth ofthick-oxide transistors. Thus, the first amplification path 24(including the first amplifier 28) is the relatively higher-bandwidthamplification path, at least in comparison to the second amplificationpath 32, of the amplifier circuit 20.

The first amplifier 28 can have a variety of different circuitconfigurations while having thin-oxide output transistors and thin-oxideother transistors throughout. The first amplifier 28 can be either avoltage amplifier or a current amplifier. As a voltage amplifier, thefirst amplifier 28 can be, e.g., an operational amplifier. As a currentamplifier, the first amplifier 28 can be, e.g., a transconductor or anoperational transconductance amplifier. The first amplifier 28 can alsobe any of fully single-ended (i.e., receiving a single ended signal atan input 56 and producing a single-ended signal at the output 60), fullydifferential (i.e., receiving a differential signal at the input 56 andproducing a differential signal at the output 60), or partiallysingled-ended, partially differential (i.e., have a single-ended input56 and a differential output 60, or have a differential input 56 and asingle-ended output 60). The schematic depiction of signal paths in FIG.1 can represent either single-ended or differential signal paths.

FIG. 2 depicts an exemplary embodiment of the first amplifier 28 a. Thedepicted embodiment implements a fully-differential transconductor,having a differential input 56 a, 56 b accepting a differential inputvoltage and a differential output terminal 60 a, 60 b producing adifferential output current, and can potentially be used in a number ofdifferent applications that call for transconductors, including Gm-Cfilter configurations. As indicated above, however, the first amplifier28 can have a number of different configurations and functional roles.In FIG. 2, the first amplifier 28 a includes four thin-oxide outputtransistors, including two thin-oxide NMOS output transistors 48 a andtwo thin-oxide PMOS output transistors 48 b, having drains connected tothe differential output terminal 60 a, 60 b of the first amplifier 28 a.The thin-oxide output transistors 48 a, 48 b have gates coupled to thedifferential input terminal 56 a, 56 b of the first amplifier 28 a. Thefirst amplifier 28 can also include other circuitry besides thethin-oxide output transistors. In the embodiment of FIG. 2, the firstamplifier 28 a also includes a plurality of NMOS and PMOS thin-oxidetransistors configured in a common-mode and biasing portion 64 of thecircuit to provide common-mode and biasing control connections 64 a, 64b, 64 c to other portions of the first amplifier 28 a.

Additionally, although the embodiment of the first amplifier 28 adepicted in FIG. 2 effectively includes only a single stage ofamplification, the first amplifier 28 can optionally include additionalstages of amplification placed between the input terminals 56 a, 56 band the output transistors 48 a, 48 b. FIGS. 3A, 3B, and 3C depictexemplary embodiments of such additional stages of amplification,including, respectively, differential pairs 68 of thin-oxide NMOStransistors (or, alternatively to that shown, thin-oxide PMOStransistors), pseudo differential pairs 72 of thin-oxide NMOStransistors (or, alternatively to that shown, thin-oxide PMOStransistors), or complementary pseudo differential pairs 76 ofthin-oxide NMOS and PMOS transistors.

The second amplifier 36 can be configured to include thick-oxide outputtransistors (e.g., thick-oxide NMOS and PMOS output transistors 80 a, 80b shown in FIG. 4). The second amplifier 36 can include at least one,and optionally a plurality of thick-oxide output transistors, andoptionally every output transistor of the second amplifier 36 can be athick-oxide output transistor. The output transistors of the secondamplifier 36 have at least drains connected to an output 88 of thesecond amplifier 36, and are the only transistors of the secondamplifier 36 having drains connected to the output 88 of the secondamplifier 36. The configuration of the second amplifier 36 to have onlythick-oxide output transistors enables the second amplifier 36 to be arelatively high output-voltage-swing amplifier in comparison to asimilar amplifier having thin-oxide output transistors. The secondamplifier 36 can include either thick- or thin-oxide transistors inremaining portions of the amplifier 36, beside the output transistors,as the design of the amplifier 36 may dictate. For example, to preservethe advantage of the second amplifier 36 being a highoutput-voltage-swing amplifier, the second amplifier 36 can optionallyinclude thick-oxide transistors at any stage of the amplifier 36 atwhich amplification of a signal traveling through the amplifier 36exists to such an extent as to surpass the voltage-withstandingcapability of thin-oxide transistors. Thus, the second amplifier 36 caninclude a selected mix of thick- and thin-oxide transistors, so long asat least the output-transistors are thick-oxide transistors. Thus, thesecond amplifier 36 is able to take full advantage of the higheroutput-voltage swing of thick oxide transistors, and not suffer effectsfrom the lower output-voltage swing of thin-oxide transistors. In themulti-path, multi-oxide-thickness amplifier circuit 20, the secondamplification path 32 (including the second amplifier 36) is thereforethe relatively higher output-voltage-swing amplification path incomparison to the first amplification path 24.

The second amplifier 36, like the first amplifier 28, can have a varietyof different circuit configurations while having thick-oxide outputtransistors as the only output transistors. Similarly as discussed inregard to the first amplifier 28, the second amplifier 36 can be eithera voltage amplifier (such as, e.g., an operational amplifier) or acurrent amplifier (such as, e.g., a transconductor or an operationaltransconductance amplifier), and can be any of fully single-ended, fullydifferential, or having a single-ended input and a differential output.

FIG. 4 depicts an exemplary embodiment of the second amplifier 36 a. Thedepicted embodiment implements a fully-differential transconductor,having a differential input terminal 84 a, 84 b accepting a differentialinput voltage and a differential output terminal 88 a, 88 b producing adifferential output current. As discussed above, however, the secondamplifier 36 can have a number of different configurations andfunctional roles. In FIG. 4, the second amplifier 36 a includes fourthick-oxide output transistors, including two thick-oxide NMOS outputtransistors 80 a and two thick-oxide PMOS output transistors 80 b,having drains connected to the differential output terminals 88 a, 88 bof the second amplifier 36 a. The thick-oxide output transistors 80 a,80 b are the cascodes of, and have sources connected to, four additionaltransistors, including two NMOS and two PMOS transistors 92 a, 92 b,which have their gates connected to the differential input terminals 84a, 84 b of the second amplifier 36 a, and receive a voltage input fromthe input terminals 84 a, 84 b and feed a current signal to thethick-oxide output transistors 80 a, 80 b. In the depicted embodiment,the additional NMOS and PMOS transistors 92 a, 92 b incascode-configuration with the thick-oxide output transistors 80 a, 80 bcan be selected to be thin-oxide transistors because they are notsubjected to the full output-voltage swing of the amplifier 36 a duringits operation. The second amplifier 36 can also include other circuitrybesides the thick-oxide output transistors 80 a, 80 b and the additionalcascode-configuration transistors 92 a, 92 b. In the embodiment of FIG.4, the second amplifier 36 a also includes a plurality of further NMOSand PMOS transistors configured to provide various biasing, common-modecontrol, and further amplification during the operation of the depictedfully differential embodiment of the second amplifier 36 a. Some of theadditional transistors of the second amplifier 36 a can be selected toalso be thick-oxide transistors. In FIG. 4, additional transistors 96 a,96 b having their gates connected to the gates of the thick-oxide outputtransistors 80 a, 80 b are also selected to be thick-oxide transistorsbecause they are potentially subject, at least to some degree, to avoltage swing having a magnitude similar or related to the magnitude ofthe output-voltage swing. All the other transistors, not yet discussed,of embodiment of FIG. 4 are thin-oxide transistors.

Thus, in some embodiments, the second amplifier 36 has several differentcategories of transistors, including a first category of thick-oxideoutput transistors (e.g., thick-oxide NMOS and PMOS output transistors80 a, 80b), a second category of other thick-oxide transistors (e.g.,thick-oxide NMOS and PMOS transistors 96 a, 96 b having gates connectedto the gates of thick-oxide output transistors, or other transistorsbeing subject to a same or similar voltage swing as the output-voltageswing of thick-oxide output transistors), and a third category ofthin-oxide transistors (e.g., transistors either not having drainsconnected to the output terminal 88, not having gates connected to thegates of thick-oxide output transistors, or not being subject to a sameor similar voltage swing as the output-voltage of the thick-oxide outputtransistors; such as cascode-configuration NMOS and PMOS transistors 92a, 92 b).

As with the first amplifier 28, the second amplifier 36 can includefurther amplification stages as may be desirable for variousapplications, and thus the second amplifier 36 could be expanded byadding the exemplary embodiments of additional amplification stagesdepicted in any of FIGS. 3A, 3B, and 3C. When adding additionalamplification stages to the second amplifier 36, however, possibly incontrast to the addition of additional amplification stages to the firstamplifier 28, the added transistors can potentially be selected to beeither thick-oxide or thin-oxide transistors depending on the degree ofvoltage swing to which they are subjected. Thus, addition of thedifferential pair 68, the pseudo differential pair 72, or thecomplementary pseudo differential pair 76, as depicted in FIGS. 3A, 3B,and 3C, respectively, can be selected to be in the form of either thick-or thin-oxide NMOS or PMOS transistors.

The first amplification path 24 of the multi-path, multi-oxide-thicknessamplifier circuit 20 depicted in FIG. 1 also includes an input filter100 connected between the input terminal 40 of the amplifier circuit 20and the input 56 of the first amplifier 28, and an output filter 104connected between the output 60 of the first amplifier 28 and the outputterminal 44 of the amplifier circuit 20. The input and output filters100, 104 of the first amplification path 24 further protect thethin-oxide first amplifier 28 from voltages that can potentially exceedthe voltage-withstanding capability of thin-oxide transistors. The inputfilter 100 can be a high-pass filter that filters out DC voltages,appearing at the input terminal 40 of the amplifier circuit 20, that mayexceed the voltage-handling capacity of the thin-oxide transistors. FIG.5A depicts an exemplary embodiment 100 a of the input filter 100. InFIG. 5A, the input filter 100 a includes a capacitor C1 and a resistorR1 configured as an RC combination to provide high pass filtering in asignal-traveling direction from the left to the right in FIG. 5A (i.e.,from the input terminal 40 to the input 56 of the first amplifier 28 inthe amplifier circuit 20). The AC coupling capacitor C1 prevents DCvoltages present at the input terminal 40 of the amplifier circuit 20from reaching the input 56 of the first amplifier 28, and the resistorR1 can provide a DC bias voltage to the input 56 of the first amplifier28.

The output filter 104 filters out and prevents DC voltages, which may bepresent at the output terminal 44 of the amplifier circuit 20, and thatmay exceed the voltage-handing capacity of the thin-oxide outputtransistors of the first amplifier 28, from entering the first amplifier28 through its output 60, by either a feedback mechanism or the merepresence of such voltages at the output terminal 44 of the amplifiercircuit 20. The output filter 104 can also be a high-pass filter, whenviewed in the signal-travel direction of from the output terminal 44 ofthe amplifier circuit 20 to the output 60 of the first amplifier 28.FIG. 5B depicts an exemplary embodiment 104a of the output filter 104.In FIG. 5B, the output filter 104a includes a resistor R2 and acapacitor C2 configured as an RC combination to provide high passfiltering in a signal-traveling direction from the right to the left inFIG. 5B (i.e., from the output terminal 44 to the output 60 of the firstamplifier 28 in the amplifier circuit 20).

The input and output filters 100, 104 can be alternatively implemented,and therefore their associated benefits alternatively achieved, in someembodiments of the multi-path, multi-oxide-thickness amplifier circuit20. The input and output filters 100, 104 can be alternativelyimplemented through the re-use of parasitic or inherent capacitances andresistances of circuit elements of the first amplifier 28 connected tothe input and output 56, 60 of the first amplifier 28. For example, theresistor R2 of the output filter 104 can optionally, instead of being adistinct resistor in its own right, be implemented by the outputresistance of the output transistors of the first amplifier 28, and insuch cases a separate resistor of the output filter 104, distinct fromthe first amplifier 28, is not needed.

The input filter 100 can optionally be omitted, and instead one canprevent the presence of relatively large voltage magnitudes at the inputterminal 40 of the amplifier circuit 20 through circuit and systemdesign choices.

The multi-path, multi-oxide-thickness amplifier circuit 20 can alsoinclude parallel amplification paths in addition to the first and secondamplification paths 24, 32 (having the first and second amplifiers 28,36). In general, the amplifier circuit 20 can include a plurality ofamplification paths, each having an amplifier that includes outputtransistors having a different oxide thickness than the other amplifiersof the other amplification paths. Thus, each amplifier (andcorresponding amplification path) can achieve a different combination ofbandwidth and output-voltage swing capability. Such an embodiment can betailored to very precisely meet gain (as related to the achievedbandwidth) and output-voltage swings over a broad range of operationalfrequencies for any given application.

FIG. 6 depicts one embodiment of the multi-path, multi-oxide-thicknessamplifier circuit 20 a having the plurality of parallel amplificationpaths and amplifiers. The depicted embodiment includes at least threeseparate amplification paths 24, 32, 108 and corresponding amplifiers28, 36, 112, but, as the dashed-line portion of the schematic indicates,additional amplification paths and corresponding amplifiers can also beincluded. In addition to the first amplification path 24 having thefirst amplifier 28 (which includes only thin-oxide output transistorsand thin-oxide other transistors), and the second amplification path 32having the second amplifier 36 (which includes thick-oxide outputtransistors), the embodiment of FIG. 6 includes at least a thirdamplification path 108 having a third amplifier 112, which includesmedium-oxide output transistors having a medium oxide-thickness (incomparison to the thin and thick oxide thicknesses). The third amplifier112 can include medium-oxide output transistors, and othermedium-oxide-thickness transistors as well as thin-oxide transistorsconfigured according to a similar methodology to that discussed inregard to the placement of thick-oxide transistors in the secondamplifier 36. For example, in the depicted embodiment, every outputtransistor of the third amplifier 112 is a medium-oxide thicknesstransistor, and any other transistor in the third amplifier that needsto withstand a medium-level voltage swing can be selected to be amedium-oxide transistor, etc. Relative to the first amplifier 28, thethird amplifier 112 can withstand a higher output-voltage swing, but hasa reduced bandwidth. Relative to the second amplifier 36, the thirdamplifier 112 can only withstand a reduced output-voltage swing, but hasa greater bandwidth.

In FIG. 6, the first and third amplification paths 24, 108 include inputand output filters 100 a, 100 b, 104 a, 104 b. (As discussed above,however, in other embodiments, input filters 100 can be omitted, andoutput filters 104 can be implemented at least in part through re-use ofamplifier output resistances.) In FIG. 6, the input and output filters100 b, 104 b of the third amplification path 108 are arranged similarlyto those of the first amplification path 24. Further, in the depictedembodiment, the input and output filters 100 b, 104 b of the thirdamplification path 108 have the same general configuration as shown inthe exemplary embodiments of FIGS. 5A and 5B, although it is possible tovary the actual resistance and capacitance values as appropriate toachieve desired bandwidth properties for the third amplification path108. However, as also similarly discussed above in regard to the firstamplification path 24, the input and output filters 100 b, 104 b of thethird amplification path 108 can take a variety of forms. Additionally,the interconnection of the plurality of parallel amplification paths canbe adjusted from that shown in FIG. 6 to that of another exemplaryembodiment of the amplifier circuit 20 b depicted in FIG. 7. In FIG. 7,the interconnection of the first and third amplification paths 24 b, 108b occurs in such a way as to effectively re-use portions of the outputfilter 104d of the third amplification path 108b as part of the outputfilter 104 c of the first amplification path 24 b. That is, thestaggered interconnection depicted in FIG. 7 enables a potentially moreadvantageous selection of resistance and capacitance values (i.e.,potentially physically smaller and occupying less chip area) for theoutput filters 104 c, 104 d of the first and third amplification paths24 b, 108 b.

FIGS. 8A and 8B are simplified cross-sectional depictions of exemplaryembodiments of thin- and thick-oxide transistors 116, 120, respectively,suitable for use in the multi-path, multi-oxide thickness amplifiercircuit 20 wherever thin- and thick-oxide transistors are used (i.e., asthin-oxide output transistors, thin-oxide other transistors, thick-oxideoutput transistors, thick-oxide other transistors, etc.). FIG. 8Adepicts an exemplary embodiment of an NMOS thin-oxide transistor 116having N-type drain and source regions 124, 128 formed in a p-type bodyregion 132. The depicted thin-oxide transistor 116 includes a gate-oxidelayer 136 separating a gate 140 from the body 132, over the region ofthe body 132 that becomes a conducting channel during circuit operation.The gate-oxide layer 136 has a first gate-oxide thickness 144. Thegate-oxide thickness 144 of the thin-oxide transistor 116 is typicallythe thinnest gate oxide formed on a particular chip, or capable of beingformed by a particular integrated circuit process. FIG. 8B depicts anexemplary embodiment of an NMOS thick-oxide transistor 120 having N-typedrain and source regions 148, 152 formed in a p-type body region 156.The depicted thick-oxide transistor 120 includes a gate-oxide layer 160separating a gate 164 from the body 156 over the region of the body 156that becomes a conducting channel during circuit operation. Thegate-oxide layer 160 has a second gate-oxide thickness 168. The secondgate-oxide thickness 168 of the thick-oxide transistor 120 is at leastgreater than the first gate-oxide thickness 144 of the thin-oxidetransistor 116 depicted in FIG. 8A. In some embodiments, the secondgate-oxide thickness 168 can be substantially greater than the firstgate-oxide thickness 144. For example, the second gate-oxide thickness168 can optionally be either one-and-a-half or twice as great as thefirst gate-oxide thickness 144. Other ratios of the second 168 to firstgate-oxide thicknesses 144 are also possible. Furthermore, in anembodiment including at least three amplification paths havingamplifiers including, respectively, thin-, medium- and thick-oxidetransistors, each level of gate-oxide thickness can be at least greaterthan the preceding level of gate-oxide thickness. The different levelsof gate-oxide thicknesses can also be related to each other bygate-oxide thickness ratios, such as one-and-a-half to one, or two toone, for successively thicker gate-oxide layers (e.g., a medium-to-thinratio, and a thick-to-medium ratio having these exemplary values).

FIG. 9 depicts an exemplary embodiment of frequency responses (i.e.,gain plotted as a function of frequency) of the first amplifierindividually 172, the second amplifier individually 176, and themulti-path, multi-oxide-thickness amplifier circuit altogether 180.

In FIG. 9, the first amplifier 28 can contribute to the overallamplifier circuit frequency response 180 at higher frequencies to agreater degree than the second amplifier 36, and, conversely, the secondamplifier 36 can contribute to the overall amplifier circuit frequencyresponse 180 at lower frequencies to a greater degree than the firstamplifier 28. In one embodiment, this differing contribution to theoverall frequency response 180 of the amplifier circuit 20 by the firstand second amplifier circuits 28, 36 can be used advantageously to allowfor relaxing of the stability requirements of the second amplifier 36.

Thus, the second amplifier 36 can optionally be designed to be unstableby itself (thus relaxing various design burdens), but be included in acombination with the first amplifier 28 than produces and embodiment ofthe multi-path, multi-oxide-thickness amplifier circuit 20 that isstable collectively. This can allow the second amplifier 36 to bedesigned to have a wider bandwidth than would otherwise be possible ifit was required that the second amplifier 36 be stable on its own.

In operation, the multi-path, multi-oxide thickness amplifier circuit 20can be configured to receive different power supply voltages, and toprovide the different power supply voltages to different internalportions of the amplifier circuit 20. For example, the amplifier circuitcan be configured to receive a first, lower power supply voltage andprovide that first power supply voltage to portions of the amplifiercircuit having either thin- or thick-oxide output transistors, as wellas receive a second, higher power supply voltage and provide that secondpower supply voltage to portions of the amplifier circuit 20 having onlythick-oxide output transistors. This concept can also be extended toembodiments of the amplifier circuit 20 having more than two parallelamplification paths, such as the embodiments depicted in FIGS. 6 and 7.In such embodiments, three or more power supply voltages can be receivedby the amplifier circuit 20 and delivered to portions of the amplifiercircuit according to a matching of increasing power supply voltagelevels to increasing gate-oxide thickness.

Other embodiments of the multi-path, multi-oxide-thickness amplifiercircuit 20 are also possible. For example, aspects and components ofvarious embodiments described herein can also be combined and mixed witheach other to create new embodiments of the amplifier circuit.

1. An amplifier circuit, comprising: a first amplifier having at leastone thin-oxide output transistor; an output filter connected between thefirst amplifier and a common output terminal, wherein the output filterimplements high-pass filtering from the output terminal to an output ofthe first amplifier and includes a capacitor connected between theoutput terminal and the output of the first amplifier; and a secondamplifier having at least one thick-oxide output transistor; wherein thefirst and second amplifiers are respectively included in first andsecond parallel amplification paths connected between a common inputterminal and the common output terminal, and the thin-oxide outputtransistor has a gate-oxide layer thickness that is less than agate-oxide layer thickness of the thick-oxide output transistor. 2-3.(canceled)
 4. The amplifier circuit of claim 1, wherein the gate-oxidelayer thickness of the thin-oxide output transistor is less thanone-half the gate-oxide layer thickness of the thick-oxide outputtransistor.
 5. The amplifier circuit of claim 1, wherein the firstamplifier is configured to receive and provide a first power supplyvoltage to a circuit branch containing the at least one thin-oxideoutput transistor, the second amplifier is configured to receive andprovide a second power supply voltage to a circuit branch containing theat least one thick-oxide output transistor, and wherein the first powersupply voltage is less than the second power supply voltage.
 6. Theamplifier circuit of claim 1, wherein every output transistor of thefirst amplifier is a thin-oxide transistor and has a drain connected toan output of the first amplifier; and wherein every output transistor ofthe second amplifier is a thick-oxide output transistor and has a drainconnected to an output of the second amplifier.
 7. The amplifier circuitof claim 1, wherein every transistor of the first amplifier is athin-oxide transistor.
 8. The amplifier circuit of claim 1, wherein thesecond amplifier contains at least one thin-oxide transistor.
 9. Theamplifier circuit of claim 1, further comprising an input filterconnected between the input terminal and the first amplifier, whereinthe input filter implements high-pass filtering from the input terminalto the first amplifier, and includes a capacitor connected between theinput terminal and an input of the first amplifier.
 10. The amplifiercircuit of claim 1, wherein the first and second amplifiers are at leastone type of amplifier selected from the group consisting of a voltageamplifier, an operational amplifier, a current amplifier, atransconductor, and an operational transconductance amplifier.
 11. Theamplifier circuit of claim 1, wherein the first and second amplifiersare at least one type of amplifier selected from the group consisting ofa fully single-ended amplifier, a fully differential amplifier, asingle-ended-input differential-output amplifier, and adifferential-input single-ended-output amplifier.
 12. The amplifiercircuit of claim 1, wherein the second amplifier is configured to beunstable if operated by itself, and the parallel combination of thefirst and second amplifiers is configured to be stable.
 13. An amplifiercircuit, comprising: a first amplifier having at least one thin-oxideoutput transistor; a second amplifier having at least one thick-oxideoutput transistor, wherein the first and second amplifiers arerespectively included in first and second parallel amplification pathsbetween a common input terminal and an common output terminal, and thethin-oxide output transistor has a gate-oxide layer thickness that isless than a gate-oxide layer thickness of the thick-oxide outputtransistor; and a third amplifier having at least one medium-oxideoutput transistor, wherein the third amplifier is included in a thirdamplification path connected in parallel with the first and secondamplification paths between the input and output terminals, and themedium-oxide output transistor has a gate-oxide layer thickness greaterthan a gate-oxide layer thickness of the thin-oxide output transistorand less than a gate-oxide layer thickness of the thick-oxide outputtransistor.
 14. (canceled)
 15. The amplifier circuit of claim 13,wherein the gate-oxide layer thickness of the medium-oxide outputtransistor is less than one-half the gate-oxide layer thickness of thethick-oxide output transistor, and the gate-oxide layer thickness of thethin-oxide output transistor is less than one-half the gate-oxide layerthickness of the medium-oxide output transistor.
 16. An amplifiercircuit, comprising: a first amplifier having at least one thin-oxideoutput transistor and at least one other signal-path amplifyingthin-oxide transistor; and a second amplifier having at least onethick-oxide output transistor and at least one signal-path amplifyingthin-oxide transistor that does not have its drain connected to anoutput of the second amplifier and does not have its gate connected to agate of the at least one thick-oxide output transistor; wherein thefirst and second amplifiers are respectively included in first andsecond parallel amplification paths connected between a common inputterminal and an common output terminal.
 17. The amplifier circuit ofclaim 16, wherein the thin-oxide output transistor has a gate-oxidelayer thickness that is less than a gate-oxide layer thickness of thethick-oxide output transistor.
 18. The amplifier circuit of claim 17,wherein the gate-oxide layer thickness of the thin-oxide outputtransistor is less than one-half the gate-oxide layer thickness of thethick-oxide output transistor.
 19. An amplifier circuit, comprising: afirst amplifier including at least one thin-oxide output transistor andat least one other signal-path amplifying thin-oxide transistor; asecond amplifier including at least one thick-oxide output transistorand at least one signal-path amplifying thin-oxide transistor that doesnot have its drain connected to an output of the second amplifier anddoes not have its gate connected to a gate of the at least onethick-oxide output transistor, wherein the first and second amplifiersare respectively included in first and second parallel amplificationpaths connected between a common input terminal and a common outputterminal; and a third amplifier having at least one medium-oxide outputtransistor, wherein the third amplifier is included in a thirdamplification path connected in parallel with the first and secondamplification paths between the input and output terminals, wherein themedium-oxide output transistor has a gate-oxide layer thickness greaterthan a gate-oxide layer thickness of the thin-oxide output transistorand less than a gate-oxide layer thickness of the thick-oxide outputtransistor. 20-24. (canceled)
 25. An amplifier circuit, comprising: afirst amplifier having a plurality of thin-oxide output transistors; anda second amplifier having a plurality of thick-oxide output transistors,wherein the first and second amplifiers are respectively included infirst and second parallel amplification paths connected between a commoninput terminal and an common output terminal, and the thin-oxide outputtransistors have gate-oxide layer thicknesses that are less thangate-oxide layer thicknesses of the thick-oxide output transistors. 26.The amplifier circuit of claim 25, wherein the first amplifier provideshigher-bandwidth amplification than the second amplifier, and the secondamplifier provides a higher output-voltage swing than the firstamplifier.
 27. The amplifier circuit of claim 25, wherein the firstamplifier is configured to receive and provide to the thin-oxide outputtransistors a first supply voltage, the second amplifier is configuredto receive and provide to the thick-oxide output transistors a secondsupply voltage, and the second supply voltage has a greater magnitudethan the first supply voltage.
 28. The amplifier circuit of claim 25,further comprising an output filter connected between the firstamplifier and the output terminal, wherein the output filter implementshigh-pass filtering from the output terminal to an output of the firstamplifier, and includes a capacitor connected between the outputterminal and the output of the first amplifier.